Hello everyone,
I think I have done quite a good work tonight in order to balance spellbook during creation. They are not divided into elements anymore but by kind of spells. Let me explain the new schools.
Destruction (thank you morrowind for the name) cost : 10
Every direct damage spells.
HurlBoulder
Fireball
MeltingTouch
Infernal
MorrigansSpite
FlameDart
Icebolt
IceStorm
ChainLightning
LightningStrike
I greatly decreased cost of every spell (they now cost from 1 to 3 mana to be cast). Combat are more dynamic like this and I find it natural to have to wait from 1 to 3 turn in order to cast one direct damage spell.
Time Control (It was hard to find a name and I am not sure it's a good one but not so bad either) cost : 10
Whirlwind
TitansBreath
 Reveal
 Quicken
 GreaterEndurance
 Haste
 StinkingMud
I also decreased costs to make more use of these spells, reveal now costs only 1 making this school perfect for exploration. And I buffed every spell a bit
Conjuration (invocation from previous elements) cost : 10
Now every invocation will last 30 turns and won't be permanent anymore (except familiar which is logical), it's quite a long period but prevent from having a main force of only invocation (I find it fair, maybe mistaking)
SummonStoneGiant
SummonEarthElemental
 SummonIceLord
 SummonWaterElemental
 SummonAirElemental
 SummonFireGiant
 SummonFireElemental 
Alteration (my favorite one) cost: 10
Every spell which have an effect on elements or on champions, sovereigns, ...
I also made spell costs cheap in order to cast more spells and have more fun 
RaiseLand
 LowerLand
 MakeTundra
 CreateDesert
 FreezeSettlement
 Volcano
 GreaterRaiseLand
 StoneSkin
 FreezeEnemy
 Deflect
 BurningBlade
 ShieldOfFire
 RockSlide 
Other schools changes
I changed a lot of duration, effects, and spells costs to make more use of spells and I find my tonight games especially fun. And you can really choose only one school and don't be afraid of the shards you will see in your game. Moreover choosing only one school make it possible to add more point in other part of creation.
Everything is almost completly tested and myself approved, I think I will give you all the modified .xml tomorrow.
Every spells I changed were modified according to my feelings and not really rationnaly but I think it's not bad balanced but I expect and want some optimizing people to give commentaries on my change. 
ps : I made an Elric of Melniboné tonight with only alteration school and developing just adventure and a bit of magic and warfare tree. He had so much alteration and good equipment on him that I conquered two AI just with him. (just in case someone of stardock read this post, protect friend spell is not working and freezeEnemy works too much  )
 )